Real black powder can be weighed or measured volumetrically with a powder measure calibrated in grains.

It's the substitutes that must be measured volumetrically, because when they say "max load is xx grains", they don't mean it literally. They mean the amount of black powder substitute measured by a black powder measure set to xx grains. The weight amount will be considerably lower.

All the black powder substitutes are less dense than black powder so 100 grains by volume might only be 70 grains by weight more or less depending on exactly which sub you are using.

There's nothing wrong with using a scale to measure out precise amounts of subs as long as you remember they don't mean xx grains literally, in fact, BH 209 loads for cartidges are given by weight while the loads for muzzleloaders are listed by volume.